MST_K07_CL - SPEED REGULATOR FOR UNIVERSAL MOTORS WITH TORQUE CONTROL



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INTRODUCTION

The MST_K07_CL is a regulator for universal motors (brushes) at 220V that allows to control the torque or to keep the rotation speed constant at the fixed value as the load varies. It is designed to work with washing machine motors equipped with a tachometer (or speed) sensor such as the MCA 30/64 motor. Figure 1 shows the photo of the controller assembled in the PCB version.


microst: MST_K07_CL

Fig. 1 -  MST_K07_CL assembled

DESCRIPTION OF THE REGULATOR

The MST_K07_CL regulator is the version with automatic torque control of the MST_K07 universal motor regulator: the motor is adjusted so that the rotation speed remains constant at the set value as the load varies. If the load applied to the motor tends to reduce / increase the rpm then the regulator will react by bringing the rpm value back to the set one. This type of regulation is obtained through the closed loop control that the regulator implements on the motor. This control is based on the electrical signal coming from the speed sensor which converts the rotation of the motor shaft into a proportional electrical voltage. As a reference we take washing machine motors (such as the MCA 30/64) in which there is a tachometer sensor, coaxial to the motor, which supplies an alternating voltage of amplitude proportional to the rotation speed of the motor shaft. The MST_K07_CL regulator, suitable for these types of motor, reads this voltage and compares it with a voltage, set via a potentiometer, which represents the setting of the rotation speed value to be adjusted. The regulator will then power the motor so that the voltage value coming from the sensor, suitably conditioned, remains equal to the voltage value set by the potentiometer. To do this, the MST_K07_CL regulator uses an 8-bit microcontroller, in which a firmware is loaded that implements a digital PID regulator. The digital PID controllers then compare the digital signals that represent the inputs (reference and feedback) and generate an output signal which is the input of the power actuator. Specifically, the actuator stage consists of the TRIAC piloted with the ignition angle variation technique. In general, a TRIAC at the zero crossing of the voltage present at its A1 and A2 anodes switches off and on again when a current pulse is present on the GATE terminal. By changing the instant of time in which the pulse is given on the GATE terminal with respect to the previous zero crossing (it is said that the phase angle is changed) the average voltage that powers the load is varied as only a portion of the half-wave of voltage is supplied to the load. So if you choose a zero ignition time (0 ) the whole half-wave goes to the load and in this case you have the maximum power supply and therefore the maximum motor speed. If you choose a time equal to half of the half wave (90 ) only half of the sine wave will go to the load and you have a 50% adjustment. Finally, if the TRIAC is turned on a few moments before the next zero crossing (180 ), no voltage will go to the load and there is a regulation of 0%.
The MST_K07_CL circuit works at the internal voltage of 5V obtained from the mains voltage through a transformless circuit or without a transformer.

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